In systems with communication functions, antennas for transmitting and receiving radio waves are required.
In such cases, an antenna can be housed within a system casing together with other functions of the system, or can be provided outside of the system casing. In order to reduce the size of a system, it is preferable for the antenna to be stored within the system casing.
If the antenna is housed within the system casing, there are two significant problems. The first problem is reducing the size of the antenna. The second problem is protecting the electronic circuits of the system from electromagnetic waves emitted from the antenna.
The size of the antenna depends on the frequency of the radio waves which are transmitted and received by the antenna. Thus, if the frequency is low, it is difficult to reduce the size of the antenna. On the other hand, if the frequency is relatively high, for example, in the case of a high frequency of 100 MHz or more, the required antenna size becomes smaller. Consequently, the possibility of the antenna to be housed within the system casing increases, and it is thought that the first problem can be solved.
Thus, it becomes important to protect the electronic circuits from the electromagnetic waves emitted from the antenna, which is the second problem. In other words, it becomes important to suppress a malfunction of the electronic circuits because of the electromagnetic waves emitted from the antenna.